The mechanisms governing the movements of chloride across biological membranes are to be studied in three systems: (1) squid giant axon, where Cl appears to be actively transported inwards, (2) barnacle giant muscle fibers, where no electrochemical gradient for Cl appears to exist, and (3) Aplysia giant neurons, where active Cl extrusion takes place. All of these systems will be examined for a relationship between internal pH regulation and the transmembrane movements of Cl. BIBLIOGRAPHIC REFERENCES: Russell, J. M. 1976. ATP-Dependent Chloride Influx into Internally Dialyzed Squid Giant Axons. Biophys. J., 16: 156a. Russell, J. M. and Brodwick, M. S. 1976. Chloride Fluxes in the Dialyzed Barnacle Muscle Fiber and the Effect of SITS. Biophys. J., 16: 156a.